Staphylococcus aureus is an opportunistic bacterial pathogen involved in severe infections in humans. S. aureus pathogenicity islands (SaPIs) are mobile genetic elements that carry genes encoding superantigen toxins and other virulence factors, and are mobilized at high frequency by specific ?helper? bacteriophages. Many SaPIs redirect the assembly pathway of their helpers to form capsids that are smaller than those normally made by the phage. This request is for a diversity supplement to R01 AI083255 (parent grant) to support Ms. N'Toia Hawkins, a graduate student in my lab who is currently funded by a UAB diversity fellowship that expires in July 2019. The parental R01 project is focused on understanding the mobilization and size redirection process for one class of SaPIs?including SaPI1 and SaPIbov1?that are mobilized by headful packaging helper phages with isometric capsids, such as 80?. We recently described a new class of SaPIs (including SaPIbov5) that are mobilized by prolate cos phages, such as ?12. The mechanism of capsid size redirection is completely different from the SaPIs that are mobilized by headful packaging phages, and depends on a SaPI-encoded capsid protein (CP) homolog called Ccm. The overall objective of the N'Toia's project is to understand the mechanism of Ccm-mediated capsid size redirection and, more broadly, the mobilization of cos type SaPIs. We propose to incorporate this project as a supplemental aim to the parent R01 with N'Toia taking the lead, with funding from the proposed diversity supplement. The supplement project has three specific aims: Aim 1: Determine structures of ?12 (large) and SaPIbov5 (small) capsids Aim 2: Identify the location of Ccm in SaPIbov5 capsids Aim 3: Define the role of the CP and Ccm N-terminal domains in the size redirection process These aims will be addressed by a combination of genetics, biochemistry, cryo-EM and three-dimensional reconstruction approaches, and will lead to new insights into the process of prolate phage assembly and genetic mobilization in S. aureus. Furthermore, the project will serve as an excellent vehicle for Ms. Hawkins' training and development into an independent scientist and to help her reach her career goals.